1. Field of the Invention
This invention relates to a process for rotolining with tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer as the rotolining polymer and to rotolining compositions.
2. Description of Related Art
J. Scheirs, Modern Fluoropolymers, John Wiley & Sons (1997) describes the rotolining process, which involves the adding of sufficient fluoropolymer in powder form to a steel vessel to coat the interior surface of the vessel with the desired thickness of the fluoropolymer, followed by rotating the vessel in three dimensions in an oven, to melt the fluoropolymer, whereby the fluoropolymer covers the interior surface of the vessel and forms a seamless lining (p. 315). The resultant fluoropolymer lining protects the vessel from corrosive materials stored or handled by the vessel, by virtue of the chemical inertness of the fluoropolymer forming the lining and the lining being continuous with respect to the interior surface of the vessel that would be exposed to the corrosive materials if the lining were not present. Thus, the lining is free of holes, even pinholes, through which the corrosive material could penetrate the lining to attack the material of construction of the vessel.
J. Scheirs also clarifies the relationship between the vessel and the fluoropolymer lining. For some fluoropolymers, notably copolymer of ethylene with either tetrafluoroethylene (ETFE) or chlorotrifluoroethylene (ECTFE) and polyvinylidene fluoride (PVDF), the lining adheres to the interior surface of the vessel, while for the perfluorinated melt processible polymers, tetrafluoroethylene/hexafluoropropylene (FEP) and tetrafluoroethylene/perfluoro(alkyl vinyl ether) (PFA), such polymers form only a loose lining within the vessel (p. 314). The PFA available for rotolining has been tetrafluoroethylene/perfluoro(propyl vinyl ether). The loose lining is held in place by the configuration of the interior surface of the vessel, i.e. mechanically locked into place, to provide the necessary protection to the vessel. The reason why the lining is loose arises from the high shrinkage of the perfluorinated polymer when the lined vessel is cooled from the rotolining operation, the shrinkage of PFA even exceeding that of FEP, causing the lining to separate from the interior surface of the vessel. While this is satisfactory in some applications, the lack of adhesion between lining and interior surface of the vessel becomes disadvantageous in such vessels as pipes, wherein the opportunity for mechanical restraint on movement of the lining is limited, especially as the length of the pipe increases. The passage of corrosive material, such as oil through the pipe, especially when the flow and/or temperature of the material varies, subjects the lining to mechanical stress, which can cause the lining to crack and fail.
Japanese Patent 2904593, first published as Kokai H4-267744 on Sep. 24, 1992, discloses the rotolining of a vessel using PFA which bubbles during rotolining and solves the bubbling problem by adding from 0.1 to 30 wt % of a fine powder, disclosing inorganic powder or metal powder such as glass, silicon, zinc, aluminum, copper or the like, to the PFA. The preferred amount of fine powder is 5 wt %, and the resultant lining is 2.0 mm (80 mils) thick. No effect on adhesion of the rotolined coating is disclosed. The effect of the fine powder is disclosed, however, viz. to cause the bubbles to adhere to the freely moving fine powder particles so as to be released to the outside, whereby the gas bubbles do not remain in the coating. The patent discloses that the presence of the fine powder, however, causes another problem, namely that the fine powder can deposit on the surface of the coating to become a contaminant in the chemical stored in the vessel. The patent solves this problem by applying a rotolined second layer onto the lining (first layer), with the second layer being free of fine powder. The second layer is kept thinner than the first layer so that the formation of bubbles in the second layer is small, and the thickness of the second layer is disclosed to be 0.5-1.0 mm (20-40 mils).
Just as the first layer in the Japanese patent must be thick enough to insure that the underlying interior surface of the vessel is protected from the chemical stored in the vessel, so must the second layer be thick enough to insure that the chemical stored in the vessel does not come into contact with the fine powder present in the first layer. Unfortunately, a 1.0 mm second layer thickness is too thin to insure that the first layer is completely covered by the second layer. Rotolinings are typically uneven in thickness, because the uniformity of the deposition of the PFA powder on the surface being coated is uncontrollable.
The problems of preventing additives to the PFA first layer (undercoat) from escaping to the interior of the rotolined article and adhering the PFA rotolining to the interior surface of the article being lined remain to be solved.